Light-emitting diodes (LEDs) are widely used as a semiconductor lightning source. One of the methods of constructing an electronic circuit using an LED is surface-mount technology also known as chip-on-board (COB) technology, in which the LED is mounted directly on a printed circuit board (PCB). In COB devices, an LED die is supplied without a package and is attached directly to a circuit board. The LED die is then wire bonded and protected from mechanical damage and contamination by an epoxy “glob-top.”
Performance of an LED is sensitive to the temperature of the operating environment. Operation of an LED in high ambient temperatures can cause overheating and eventually device failure. One of the advantages of the COB design for LED die attachment is the enhancement of the thermal dissipation from an LED die to a thermal conductive substrate, which improves the LED lighting efficiency because LED efficiency decreases when the temperature increases.
Presently, the most popular method for mounting LEDs on PCB is a pocket design. FIG. 1 depicts a prior-art device with typical pocket-type attachment of an LED die 1 to a thermal conductive substrate 3. A dielectric layer 5 is attached on top of the substrate 3. The dielectric layer 5 has an opening 7 to form a pocket 9 where the LED die 1 is placed. The pocket 9 is formed by the walls 11 of the opening 7 and the top surface 13 of the substrate 3.
This design suffers from the significant brightness loss due to the absorption and scattering of the LED-emitted light 15. A significant portion of the light 15 emitted by the LED die 1 goes in the direction of the walls 11. When this sideways emitted light 17 hits the walls 11, it is absorbed and/or scattered by the walls 11 because of the walls' 11 optically-absorptive qualities and because the walls 11 are not sufficiently smooth to reflect the light 17.
FIG. 2 depicts another prior art device whose design also suffers from the loss of brightness. In this instance, the LED die 1 is an ultra-violet (UV) LED die. The UV LED die 1 is positioned on the bottom of the pocket 9 formed on the top surface 13 of the substrate 3. After the die 1 is placed in the pocket 9, the pocket 9 is filled with phosphor 19. When the UV light emitted by the UV LED die 1 passes through the phosphor 19, the phosphor absorbs the UV light and emits light of the visible spectrum, white light. However, since the phosphor emits the light omnidirectionally, an even larger portion of the light is absorbed and scattered by the walls 11, compared to the device of FIG. 1.